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Ultra-high precision, CNC grinding machine for single crystal diamond tool manufacture

The new PG4 enables the automated production of ultrahigh precision diamond tools

Launched in 2016, the PG4 is an automatic, ultra-high precision grinding machine designed to meet the most sophisticated of demands from toolmakers working with natural or synthetic single crystal diamond (SCD), manufactured by either high pressure high temperature (HPHT) or chemical vapour deposition (CVD).

Drawing on our 75 years of experience in designing machines for grinding and polishing diamond and related ultra-hard materials, the PG4 now enables many of the laborious and time consuming processes, such as blocking out, to be fully automated and the subsequent final radius polishing to be completed without human intervention.

The PG4 uses high quality airbearings for both wheel spindle and the pivot which are essential pre-requisites for controlled and low waviness SCD tools. Programming is extremely simple and minimal training is required.

The PG4 is an ideal machine for SCD tool manufacturers who are looking to minimise production costs and use their skilled operators in more demanding areas.

Diamond Tool Production with the PG4

The PG4 is designed to automatically manufacture standard, controlled and, under suitable conditions, low waviness diamond tools. It can be used for making facetted and convex radius tools using: natural diamond, HPHT, synthetic diamond, CVD synthetic diamond

Low and controlled waviness tools

Elliptical, parabolic, hyperbolic and blended radii profiles

Complex profile tools

Facetted, coned and radiussed indenters

Single Crystal Diamond Grinding

The Structure of Single Crystal Diamond

Like wood, diamond crystals have a 'grain', but unlike wood the 'grain' runs in many possible directions. Diamond can be ground easily as long as the grinding direction is along the 'grain' or put another way – parallel to a 'soft' direction of the crystal. The PG4 works in a very different way to a conventional grinding machine. It is useful to understand why.

Planetary Motion Wheel Spindle

To achieve the correct grinding direction, the PG4 has a wheel spindle which can be positioned in a planetary manner so that the direction of the grinding wheel's motion at the point where it meets the diamond can be aligned with the soft direction of the facet being ground. This enables rapid material removal rates to be achieved. For example, with the wheel in the '135°' position, the grinding direction is against the grain and little, if any, material will be removed. At the '270°' position, the diamond is ground rapidly as the grinding direction is now running parallel to a 'soft' direction of the crystal.

The Floating Table with Constant Grinding Load

Because diamond's hardness varies with crystal direction, it is not possible to grind diamond using a positive in-feed mechanism alone. Instead a constant force is used to simply apply a steady grinding pressure. The diamond tool is mounted on a linear motion floating table which is held in contact with the wheel by a dead weight until it meets a fixed stop. The floating table also contains the air-bearing pivot motion which enables precise radii to be formed.

Edge Perfection and Vibrations

Controlled and low waviness SCD tools can only be produced if vibrations at the wheel/diamond interface are eliminated. A composite granite base to damp vibrations and the air-bearings in the wheel and pivot spindles are all necessary to ensure that the ultimate perfect edges are achieved. Operators can also introduce vibrations as they press switches or tap the touch screen, to avoid this vibration, all aspects of human interface have been assigned to a separate tower which is connected to the main grinding machine via an umbilical harness. Once the program starts there is no further need for the operator to touch the grinding machine itself.

Unique Automatic Features

With all the primary axes under software control (pivot, wheel in-feed, wheel traverse, planetary position, and if fitted rotary axis) the PG4 introduces many new and unique possibilities for automation and exacting finesse in the grinding process

Grinding Facets – The 'Blocking Out' of a Cutting Tool Radius

Under program control, the machine will generate up to 31 facets around the radius of a tool. For each facet, the control will:

  • Rotate the wheel spindle to the required planetary position for soft direction grinding
  • Advance the wheel a controlled amount and allow the tool to grind and 'float' back to the fixed stop
  • Move the tool away from the wheel
  • Index the pivot and grind the next facet

Grinding the Radius

  • The program will then swing the pivot over the required arc to smooth the flank in conjunction with the radius
  • Hard spots 'knarts' can be identified and a sub-arc of pivot rotation selected for focussed attention
  • Once the tool is completed the wheel will move away from the tool for inspection/unloading

Simultaneous, Synchronous Axis – Grinding Vectors

The planetary , pivot rotation, wheel traverse, wheel in-feed and rotary module (RM) motions are all programmable and in the basic machine run independently of each other. Optional software is available for the PG4 such that the motion of two or more of these programmable axes can be electronically geared to run with synchronous motion. A repeating sequence of grinding directions or vectors can thus be established enabling complex profiles to be generated, such as elliptical, parabolic, hyperbolic and multiple blended radii.

Multi-Axis Configuration

The pivot and grinding wheel axes feature high precision air bearings and in normal production only these two axes would be running for the very final polishing of a low waviness radius tool.

The remaining machine axes incorporate high precision linear or rotary bearings.

The camera axis alignment is via micrometers.

Optional Fixtures and Ancillary Machines

RM76 – Rotary Axis

  • For coning, cylindrical grinding and index positioning
  • Programmable rotation to 0.001°
  • Collet chuck to suit tools up to Ø16mm
  • Other adaptors available

SAM – Metal Bond Wheels

  • High quality, porosity free
  • Extremely long life, designed for dry grinding
  • Dynamically balanced to gyroscopic tolerances

TCS1 – Tool Cooling Unit

  • Cools tool shank using sub-zero compressed air
  • Removes thermal expansion effects
  • Programmable 'on/off'

PS2B – Planetary Scaife Bench

  • For top lapping of SCD tools after radius grinding
  • Re-sharpening worn tools and general facet work
  • Adjustable planetary motion allows 'set and leave' operation

TRC – Tool Radius Check

  • Microscope with image analysis software to measure tool radius and radius waviness
  • Generates custom designed QC graphical data sheets which can then be supplied with the associated tool

Optional Software Modules

  • Synchronized pivot, planetary, traverse, infeed and rotary (RM) motions
  • Profile contour deviation

PG4 Technical Specification

Area Specification
Machine dimensions (w x d x h) 790mm x 1310mm x 1900mm
Control tower dimensions (w x d x h) 680mm x 610mm x 1910mm
Machine weight 800kg
Control tower weight 160kg
CNC control system CNC control with Intel i7 processor running Coborn .net software on a Windows platform
Grinding wheel spindle 2.0kW 0-12000 rpm programmable liquid cooled high-precision air spindle
Grinding wheel Ø 85mm Metal bond wheel or cast iron lap
Pivot spindle High-precision air spindle with radial error ≤ 0.00005mm
Lubrication Manual grease lubrication system
Process inspection Coborn integrated camera inspection system x60 to x600 magnification
Electrical requirements 380/220 3phase 50/60 Hz 16amps
Software options Rotary module, Profile contour deviation, Controlled/low waviness, Dxf. import (profile grind)
Mechanical options Rotary axis (RM) programmable, Coning spindles (CS) manual, Work piece clamping systems

Axis Description Feedback Resolution Program Resolution Travel
X Wheel head traverse 0.005mm 0.01mm 100mm
Y Wheel head infeed 0.0001mm 0.001mm 45mm
T Planetary head tilt Manual Manual 0 to 20°
B Planetary head orientation 0.01° 0.1° 360°/continuous
C Pivot spindle 0.0001° 0.001° +/- 100°
V Infeed table 0.0001mm 0.0001mm 32mm
A Rotary module (Optional) 0.0002° 0.001° 360°/continuous
TPX Fixture X Manual Manual 12mm
TPY Fixture Y Manual Manual 12mm
TPZ Fixture Z Manual Manual 75mm
TPA Fixture tilt Manual Manual -20 to + 20°
Xc Camera X 0.01mm Manual 12mm
Yc Camera Y 0.01mm Manual 12mm
Zc Camera Z Manual Manual 80mm
PG4 – Ultra-high precision, CNC grinding machine for single crystal diamond tool manufacture